1. Field of the Invention
The present invention relates to a displacement detection apparatus that detects a displacement of a surface to be measured, by means of a contactless sensor using the light emitted from a light source, and more specifically relates to a technique for detecting a displacement in a direction perpendicular to a surface to be measured.
2. Description of the Related Art
Conventionally, as the apparatus that contactlessly measures a displacement and/or shape of a surface to be measured, a displacement detection apparatus using light has been widely used. As a typical example, there is a method comprising the steps of: irradiating a surface to be measured with a laser beam; and detecting a change of the position of reflected light with a PSD. However, the method has problems that it is susceptible to the gradient of a surface to be measured, the sensitivity is low, and the resolution of measurement will decrease when the measurement range is extended.
On the other hand, there is a method using a Michelson's interferometer, with a surface to be measured as a mirror. The method has a wider detection range and is excellent in linearity, but will be affected by a change of the wavelength of a light source and a change in the refractive index of the air when the measurement range is extended.
On the other hand, there is a method (for example, see Japanese Laid-Open Patent Publication No. 1993-89480 (Patent Literature 1)) of detecting a displacement of the surface to be measured by: converging the light emitted from a light source by an objective lens on a surface to be measured; converging reflected light reflected by the surface to be measured by an astigmatic optical element; causing the converged light to enter a light receiving element to generate a focus error signal using an astigmatic method; then driving a servo mechanism using the focus error signal to displace the objective lens so that the focus position of the objective lens exists in the surface to be measured; and at this time, reading the scale mark of a linear scale that is integrally attached to the objective lens via a coupling member. The method is less likely to be affected by a change of the gradient of a surface to be measured. Therefore, the method has an advantage in that a displacement of the surface to be measured can be measured across a large measurement range with a high resolution.
In the displacement detection apparatus disclosed in Patent Literature 1, in order to increase the accuracy in detecting a displacement, the diameter of a beam to be converged on a surface to be measured is reduced by increasing the numerical aperture (NA) of the objective lens. For example, if the diameter of a beam focused on a surface to be measured is set to approximately 2 μm, then the accuracy in detecting the linear scale results in approximately several nanometers to approximately 100 plus several nanometers.